sync with MonetDB: we now require (at least) autoconf 2.57
sync with MonetDB: we now require (at least) autoconf 2.57

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb